This research examines the role of the human prefrontal cortex in the "executive" coordination of behavior. The experiments described in this proposal are focused on specific properties of the cognitive processes employed when attention is switched between two different tasks. Integral to this research will be the examination of the cortical structures involved in task switching. Specifically, the role of dorsolateral prefrontal cortex in these functions will be examined. Posterior brain regions will also be studied. It is predicted that task switching performance can be attributed to the prefrontal cortex independent of posterior brain regions. Determining the extent of prefrontal mediation in executive processes has vast implications for understanding both the nature of cognitive deficits in patients with frontal cortex damage, and how executive processes control task switching behavior in normal subjects. Task switching performance will be assessed by collecting response times when subjects alternate between two simple tasks. Control processes critical to task switching will be isolated experimentally, and the contribution of the prefrontal cortex to each will be assessed. Three experiments (see section 29.B) investigate 1) the role of prefrontal cortex in task switching, 2) how task rules are prepared for implementation, and 3) the differences between intra- dimensional, inter-dimensional, and inter-domain task switching. Each experiment examines processing components that specify a different aspect of normal cognitive performance and, to the extent that prefrontal damage affects these components, these experiments will expose the specific source of behavioral deficits in such patients.